1. Field of the Invention
The present invention relates to a photosensitive resin composition comprising an alkali-soluble resin, a quinonediazide-type photosensitive compound and a solvent, as the main components, which is sensitive to radiation such as ultraviolet rays, far ultraviolet rays, electron beams or X-rays. Further, the present invention relates to a method for forming a pattern using such a photosensitive resin composition.
2. Discussion of Background
Heretofore, a photoresist is known which comprises an alkali-soluble novolak resin and a quinonediazide-type photosensitive compound which is capable of functioning as an alkali dissolution inhibitor. With such a photoresist, at an irradiated portion, the quinonediazide-type photosensitive compound (hereinafter referred to simply as a photosensitive material) decomposes to carbene and then to a ketene, which reacts with a moisture within and outside the system to form indenecarboxylic acid, which is readily soluble in an aqueous alkaline solution. On the other hand, at a non-irradiated portion, the photosensitive material hinders solubility of the alkali-soluble novolak resin in the alkali developer (hereinafter such a hindering effect is referred to as a masking effect), whereby the non-irradiated portion is hardly soluble in an alkali developer, undergoes no substantial swelling and maintains a high film-remaining ratio. As a result, a resist pattern with a high resolution, can be obtained.
In recent years, high densification of semiconductor integrated circuits has progressed, and requirements for the production process have been increasingly severe. In order to fulfill such severe requirements, various improvements have been attempted with respect to photoresist materials, and detailed studies have been made on a wide range of materials including resins, photosensitive materials, developers and additives. Accordingly, requirements for a photoresist have also been increasingly high. In particular, high sensitivity, high resolution, high rectangularity of the pattern profile, high dry etching resistance, high heat resistance and a wide exposure margin are strongly desired.
Two types of expressions i.e. Eth and E.sub.0 are used to express the sensitivity of a photoresist, and they are, respectively, defined as follows.
Eth: The minimum exposure required to dissolve the exposed portion over a wide area (e.g. at least 2.times.2 mm) of a photoresist film coated on a substrate, completely to the substrate.
E.sub.0 : The exposure whereby, when developed by an alkali developer after exposure, a line pattern with a predetermined width (e.g. 0.6 .mu.m) on a reticle can be reproduced with the desired size (e.g. 0.6 .mu.m) on a wafer.
Eth is the minimum exposure whereby the exposed portion of a photoresist is dissolved to the substrate by development. Therefore, in order to form an image by the photoresist, an exposure of at least Eth is required. Further, in the actual process for producing semiconductors, an exposure (E.sub.0) which is of course higher than Eth is employed to attain the desired line width corresponding to the size of the mask. It is said that the exposure margin can be widened by increasing the ratio of these two types of exposure i.e. the ratio of E.sub.0 /Eth. Namely, when the exposure exceeds E.sub.0, the width of the line pattern will be smaller than the desired size, and when the exposure is less than E.sub.0, the width of the line pattern will be larger than the desired size. Further, when the exposure is gradually reduced so that the exposure becomes smaller than Eth, it becomes impossible to dissolve the exposed portion to the substrate by development. Namely, it becomes impossible to form an image. When the ratio of E.sub.0 /Eth is small e.g. at a level of 1, if the exposure actually used is slightly smaller than E.sub.0, the exposure will be less than Eth, whereby it becomes impossible to form an image, and accordingly, it will be difficult to produce semiconductors. Inversely, when the ratio E.sub.0/ Eth is large, the range of exposure for forming an image will be widened. Namely, it is thereby possible to widen the exposure margin.
On the other hand, photoresists for high resolution developed in recent years, are designed to improve the resolution and the exposure margin by increasing the masking effect by increasing the amount of the photosensitive material in the resists (e.g. Japanese Unexamined Patent Publication No. 136637/1987, Japanese Journal of Applied Physics Vol. 128, No. 10, Oct., 1989, p2110-2113). However, when the amount of the photosensitive material is increased, a decrease in the sensitivity can not be avoided. Absorption by the photosensitive material is high particularly in the i-line (wavelength: 365 nm) region which is commonly employed for semiconductor lithography in recent years. Accordingly, the difference in the energy of transmitted lights will be substantial between the upper and lower portions of the resist film coated on a wafer, whereby the pattern profile tends to be a trapezoid and can hardly be an ideal rectangle, and the resolution will hardly be satisfactory. If the amount of the photosensitive material is reduced to solve the above problems, no adequate masking effect will be obtained, and deterioration in the pattern profile, the resolution and the exposure margin will be remarkable, although the sensitivity may be improved. Thus, it is difficult to develop a high performance resist by conventional techniques, and a new concept is desired.